SYNTHESIS AND CRYSTAL-STRUCTURE OF LI2NATA7O19

The new phase Li2NaTa7O10 has been prepared by high-temperature reacti on of Na2CO3, Li2CO3 and Ta2O5. Its structure has been determined and refined, from Cu-Kalpha1, X-ray powder diffractometer data, in space g roup Pbam with unit cell: a = 15.2308(6) angstrom, b = 23.573(1) angst rom, c = 3.8366(2) angstrom, V = 1377.5 angstrom3 to R(F) = 3.1%. The structure exhibits a layered Ta7O19(3-) framework, containing TaO6 oct ahedra and TaO7 pentagonal bipyramids. The Na atoms are located in pen tagonal tunnels along the c axis; the Li atom positions were not deter mined. High-resolution electron microscopy images confirmed the basic features of the structure. Conductivity measurements showed essentiall y insulating behaviour with a limited amount of ionic conductivity at high temperatures, ca. 2 x 10(-7) OMEGA-1 cm-1 at 300-degrees-C.